Because of problems experienced by certain lines and switching equipment during digital data transmission over voiceband telephone lines, Bellcore standards now require that a matching AC impedance (on the order of 600-900 ohms) be placed across the tip and ring leads of the line to which customer provided equipment is connected for data reception. As much of the equipment currently in use does not satisfy this requirement, testing of a line to which such equipment is connected by a craftsman's test unit that is provided with such a matching impedance termination may present to the line a load impedance that is different from that presented by a customer's digital communication device, thereby impairing the ability of the craftsman to diagnose the source of a fault on the line.